and agostic configurations. The softer rotational mode in the manganese adduct leads to increased entropic contributions at all temperatures, and an increased dependence on temperature relative to the other two complexes. The near equivalence of the standard entropies of technetium and man- ganese, despite a much lower contribution from rotation of the hydrogen molecule in technetium, is due to increased contributions from other modes within the organometallic complex. The asym- metry of the rotation in the rhenium complex leads to higher contributions to the entropy than in the technetium adduct, despite the high stiffness in the mode. The much stiffer, asymmetric rotational mode in the rhenium complex, along with the higher binding enthalpy, suggest that the H2molecule may have undergone oxidative addition and dissociated to form the dihydride complex.
However, because both hydrogen atoms remain bound to the same metal center, the assumptions of the Langmuir isotherm are not violated. Effects from this transition are modeled in the Langmuir isotherm as a contribution to the site binding energy. This allows the absorption enthalpy to be modeled accurately by the Langmuir isotherm in cases of oxidative addition, but does not allow determination of whether the compound is a dihydrogen complex or a dihydride.
ECP for geometry optimization and vibrational analysis and MP2/LANL2DZ-ECP for total energy were also found to accurately describe the experimental behavior of the manganese complex and have provided additional insight into the configuration of the active binding site and the individual contributions from configurational and rotational entropy to the binding properties. Calculations for substitution of the metal center with other group 7 metals found similar trends in the thermodynamic properties to experimental reports for group 6 metals, with increasing binding enthalpy such that 5d
>3d>4d, suggesting an overall trend in properties for transition metals moving down the periodic table.
Chapter 4
Studies of [MH(η 2 -H 2 )dppe 2 ] + (M
= Fe, Ru, Os) Complexes
In the previous chapter, the Langmuir isotherm model was found to accurately describe the exper- imental behavior for hydrogen absorption onto the complex [Mn(CO)dppe2][BArF24] to form the dihydrogen complex [Mn(η2-H2)(CO)dppe2][BArF24]. Electronic structure simulations of hydrogen absorption over the fragment [Mn(CO)dppe2]+ were also successful in quantitatively reproducing the experimental behavior and thermodynamic properties, and simulations were used to extend the analysis to the other group 7 metal fragments [Tc(CO)dppe2]+ and [Re(CO)dppe2]+ to develop trends for the hydrogen absorption behavior within the periodic table. In this chapter, the analysis is extended to describe hydrogen binding to the isoelectronic organometallic fragments [MHdppe2]+ (M = Fe, Ru, Os) to form the dihydrogen complexes [MH(η2-H2)dppe2]+.
The group 8 complexes containing the cationic fragment [MH(η2-H2)dppe2]+ (M = Fe, Ru, Os) are among the most well-studied dihydrogen compounds available in the literature. The crystal structures for all three dihydrogen complexes have been determined by x-ray diffraction149–152 and found to be isostructural, and neutron diffraction experiments have been performed for the iron and ruthenium complexes to determine dihydrogen positions.149, 150 Extensive NMR work has been per- formed96to examine the dynamics of dihydrogen exchange, heterolytic cleavage, and intramolecular exchange of the hydride and dihydrogen within all three dihydrogen complexes. Multiple spectro- scopic studies have been performed to characterize the iron complex, including M¨ossbauer spec- troscopy119 and analysis of the vibrational modes through inelastic neutron scattering.110
Thermodynamic properties of the dihydrogen interaction with the metal center remain unde- termined, largely because of the high stability of the dihydrogen products and strong reactivity of the five-coordinate species in the iron and osmium compounds that prevent characterization.
While M¨ossbauer, UV-vis and IR spectroscopy have been reported for the directly synthesized five- coordinate iron complex107and H2/D2exchange has been shown to occur through a stable interme- diate,96the thermal decomposition products of the iron and osmium species have not been directly identified and have only been assigned through the analogous [RuHdppe2]+ cation, which is stable at room temperature. No characterization studies are available for [OsHdppe2]+ or [RuHdppe2]+ complexes, despite the stability of the ruthenium species.
Identification of the complexes involved in reversible hydrogen absorption is the first step re- quired for accurate thermodynamic simulations. M¨ossbauer spectroscopy, in particular, provides a probe of the valence and spin state of the central iron atom in iron complexes, as well as a measure of the charge transfer between the metal center and attached ligands. The availability of this infor- mation and the atomic selectivity of the technique makes M¨ossbauer spectroscopy uniquely suited for investigating group 8 dihydrogen complexes, and for determining mechanisms in Kubas binding.
Evidence for the identification of the thermal decomposition product of [FeH(η2-H2)dppe2][NTf2] (NTf2 = bis(trifluoromethlysulfonyl)imide) through Mossbauer spectroscopy and other techniques is presented below, and simulated thermodynamic parameters are reported for the hydrogen binding interaction in the group 8 fragments [MH(η2-H2)dppe2]+ (M = Fe, Ru, Os).
4.1 Experimental
Unless otherwise stated, all reactions were performed under a dinitrogen atmosphere using either a controlled atmosphere glovebox or Schlenk line techniques. 1,2-bis(diphenylphosphino)ethane was purchased from Strem Chemicals and used without further purification. Crystalline bis(trifluoro- methanesulfonyl)imide was purchased from Acros Organics and used without further purification.
Research-grade gases were purchased from Matheson and used directly. All solvents were dried and deoxygenated by purging with dry dinitrogen gas for 15 minutes before passing through packed
columns of activated alumina and activated copper. After synthesis, materials were stored under dry argon in an atmosphere-controlled glovebox until their use in testing.
Solution-state NMR spectra were recorded on a Varian 300 MHz instrument with 1H shifts re- ported relative to the residual solvent peak, and31P peaks reported relative to 85% H3PO4. Deuter- ated NMR solvents were purchased from Cambridge Isotopes Laboratories. Deuterated benzene was purified by vacuum distillation from a sodium/benzophenone solution before use. Deuterated bro- mobenzene was dried over CaH2 and vacuum distilled before use.
Preparation of cis-FeH2dppe2—This procedure was a modification of existing literature proce- dures.96, 153 Anhydrous FeCl2 (319.8 mg, 2.523 mmol) was suspended in 2 mL THF. A solution of 1,2-bis(diphenylphosphino)ethane (2.0052 g, 5.033 mmol) in 6 mL THF was added and the sus- pension was stirred until a milky white precipitate formed. NaBH4 (220.0 mg, 5.680 mmol) in 8 mL of dry ethanol was then added, producing strong outgassing and causing all components to immediately dissolve to form a dark red solution. The solution was stirred for 3 hours during which outgassing continued and a yellow powder precipitated. The remaining solution was decanted from the precipitate, and the precipitate was dissolved in benzene and filtered over celite to remove salt impurities. Removal of the solventin vacuo produced the product as a fine yellow powder. Yield 1.8533 g (73.5% on iron). 1H NMR spectra was broad, indicating a fluxional molecule. 1H NMR (300 MHz) in C6D6: δ -12.76 (quad., 2H); 1.80 (s, 4H); 2.14 (s, 2H); 2.40 (s, 2H); 6.44 (s, 4H);
6.58–7.12 (m, 20H); 7.14–7.80 (m, 12H); 8.67 (s, 4H). 31P NMR (300 MHz) in C6D6: δ 91.5 (s);
103.3 (s).
Preparation of [FeH(η2-H2)dppe2][NTf2]—This procedure was a modification of an existing
literature procedure.96 Bis(trifluoromethylsulfonyl)imide (327.8 mg, 1.166 mmol) in 5mL diethyl ether was added to a solution ofcis-FeH2dppe2 (1.015 g, 1.188 mmol) in 12 mL diethyl ether and allowed to stir for one hour, forming a light green precipitate. The precipitate was filtered over a medium frit and washed twice with diethyl ether. Removal of residual diethyl ether in vacuo produced the product as a light green powder. Yield 1.098 mg (81.8% on iron). Some N2 adduct was included in the product. 1H NMR (300 MHz) in C6D5Br: δ -12.61 (s, broad, 1H); -7.76 (s,
broad, 2H); 2.13 (s, 8H); 7.16–7.51 (m, 40H).31P NMR (300 MHz) in C6D5Br: δ92.0 (s).
Thermal decomposition of [FeH(η2-H2)dppe2][NTf2]—[FeH(η2-H2)dppe2][NTf2] was placed under vacuum and heated at 125◦ for 12 hours, producing an extremely reactive blue-green powder.
This powder was found to react with all available solvents, preventing direct characterization by NMR.
4.1.1 X-Ray Diffraction
Diffraction quality crystals of [FeH(η2-H2)dppe2][NTf2] were grown by layering diethyl ether over a saturated solution of the complex in benzene. The resulting yellow needles were mounted on glass fiber with Paratone oil and spectrum were recorded over 12 hours with a Bruker KAPPA APEXII x-ray diffractometer under flowing nitrogen at 298K using Mo Kαradiation.
4.1.2 M¨ ossbauer Spectroscopy
Isotopically enriched [57FeH(η2-H2)dppe2][NTf2] and its thermal decomposition products were made according to the procedures above. 57Fe metal was purchased from Isoflex USA. 57FeCl2was made by reacting shavings of 57Fe (0.13 mg, 0.228 mmol) metal with excess concentrated hydrochloric acid (1.0 mL, 12M) in 5 mL methanol. The resulting yellow solution was reduced and dried at 160◦ under vacuum for 8 hours to produce the anhydrous starting material.
Samples were loaded into a custom-built, O-ring sealed PTFE sample cell under an argon at- mosphere. Sealed samples were stable for up to 24 hours in air at room temperature. Spectra were recorded over six hours in the transmission configuration using a gas scintillation detector and source radiation from the decay of57Co in a rhodium matrix. Isomer shifts were referenced to bcc iron.
4.1.3 Computation
Electronic structure calculations were performed for the singlet state of the cationic fragments [MH(η2-H2)dppe2]+ and [MHdppe2]+ (M = Fe, Ru, Os), and the hydrogen molecule using the GAMESS-US software package.134 Geometry optimizations for the organometallic fragments were
performed using fully spin-restricted (RHF) density functional theory calculations, with the B3LYP exchange-correlation functional135, 136and the LANL2DZ basis set.137–140 An additional p polariza- tion shell for light atoms and d polarization shell for heavier atoms were added to augment the basis set. Starting structures for optimizations were taken from XRD experiments reported previously for [FeH(η2-H2)dppe2][BF4]152 and on calculated singlet structures107 for [FeHdppe2]+. Effective core potentials138–140representing the core 10 electrons for iron and phosphorus atoms, the core 28 electrons for ruthenium and the core 60 electrons for osmium were used. SCF convergence was set to 5.0 x 10−6for all calculations. Geometry optimizations were performed to a tolerance of 10−4au.
Ab initio ground state electronic energies were calculated using the spin-component scaled Møller- Plesset second order perturbation141 (SCS-MP2) scheme with the LANL2DZ-ECP basis set from the optimized geometries found from DFT calculations. Thermal corrections to the energy and normal mode frequencies were obtained from vibrational analysis of the Hessian matrix for each fragment calculated using the B3LYP/LANL2DZ-ECP level of theory with seminumerical methods, with contributions from positive and negative displacements of 0.01 Bohr. Partition functions of the fragments,qtot at 1 atm pressure were also obtained from vibrational analysis. A scaling factor of 0.96 was in calculated the thermodynamic properties and normal mode frequencies to correct known errors in the LANL2DZ basis set, consistent with common practice.143 Symmetry of the rotational modes was determined by analysis of the components of the mode eigenvector associated with the two hydrogen atoms, and the mode was considered symmetric if the x, y, and z components of the force constants for each atom were of opposite sign and deviated in value by less than 0.1 millidyne/angstrom.
The chemical potential of hydrogen gas was calculated from the partition function for the hy- drogen molecule, through the relationship:
µgas(T, P) =−kTln (qtot(T, P◦)) + ln P
P◦
(4.1)
where kis Boltzmann’s constant,T is the temperature in Kelvin,P is the pressure of the system, and P◦ is the standard pressure of the system, taken to be 1 atm. Rotational degeneracy for the
hydrogen molecule is included in the calculated partition function from the GAMESS software.
The energy for each fragment, E, was obtained as the sum of the ground-state energy,0, and the thermal correction obtained from vibrational analysis, corr(T), which contains contributions from the electronic, translational, vibrational, and rotational motions of the molecule. The binding energy of the hydrogen molecule to the organometallic site, ∆E, was calculated as:
∆E(T) =EM-H2(T)−EH2(T)−EM(T) (4.2)
where the subscripts M-H2, H2, and M represent the organometallic adduct fragment, hydrogen gas, and the bare organometallic fragment, respectively.
To obtain the M¨ossbauer parameters for the iron fragments, fullyab initiogeometry optimizations and total energy calculations were performed using the TZVP level of theory for [FeH(η2-H2)dppe2]+ and [FeHdppe2]+, with p and d polarization shells as described above. Calculations were also performed on the nitroprusside anion, [Fe(NO)CN5]2−, to be used as a reference state for isomer shifts. Isomer shift and electric quadrupole splitting values were obtained from the electronic charge density and electric field gradient at the iron nucleus according to the method described by Blaha.154 Isomer shifts were converted from the nitroprusside scale to the bcc iron scale by subtracting 0.257 mm/s.
4.1.4 Kinetic Isotherms
Kinetic isotherms were recorded for the thermal release of the H2ligand from [FeH(η2-H2)dppe2][NTf2] using a custom-built Sieverts apparatus. Solid [FeH(η2-H2)dppe2][NTf2] (257 mg) was loaded into a 14mL stainless steel reactor under an argon atmosphere and sealed before transfer to the instru- ment. Swagelok VCR copper filter gaskets with 2µm stainless steel filters were used to prevent loss of the powdered sample from the reactor during measurements. After transferring the reactor to the assembly, samples were allowed to pump down overnight to 3.1x10−7torr at the pump inlet before measurements were performed.
The rate of hydrogen release was measured by placing the sample under vacuum conditions
for nominally one hour at the desired temperature, followed by expansion of hydrogen gas from a calibrated volume into the reactor. Residual gas mass spectroscopy was performed during the evacuation of the reactor to identify the decomposition products. Ideal gas mole balance was used to determine the quantity reabsorbed after exposure of the degassed sample to 0.5 atm hydrogen gas. Time and pressure resolution of the instrument were 500 ms and 5 torr, respectively.